Conventionally, an HOV lane or the like which is used in a road system mainly seen in, for example, a North American big city is known as a road on which restrictions showing which vehicles can enter the road are imposed. An HOV lane is also called a carpool lane and is a lane in which only vehicles with two or more people are allowed to travel. A lane disposed along with normal lanes on a highway, a lane which is a shortcut of an interchange, etc. are known as HOV lanes. A road system which adopts such an HOV lane provides the user with a preferential treatment that makes it possible to arrive at his or her destination in a shorter time as long as the user travels in an HOV lane so as to prod the user to share the vehicle with one or more other persons, thereby reducing the traffic as a whole and therefore reducing traffic jams.
As a technique relating to such an HOV lane, patent reference 1 discloses a navigation device which searches for a route using information about HOV lanes as road information about a route leading to a destination set up by the user. In this navigation device, map data including road information about entrance and exit points in a highway having an HOV lane are read from a DVD and are stored in a data buffer in order to enable the user to efficiently travel in a lane on which restrictions against lane changes for entrance from and exit to another lane are imposed. A route search processing unit carries out a route search process in consideration of the enable or disable of use of an HOV lane by using the map data stored in the data buffer. When providing route guidance using an HOV lane, an HOV lane guidance unit provides guidance on a certain lane change with an image and a voice at the time that an entrance or exit point at which the user's vehicle should change its traveling direction falls within a predetermined distance from the position of the vehicle.
Further, a navigation system which provides route guidance in such a way as to enable the vehicle to economically travel by selecting a route with a small degree of fuel consumption is known (for example, refer to patent reference 2). A signal processing device in this navigation system has a unit for defining each of intersections included in each of a plurality of inputted routes as a boundary to compute the time required to travel over each section between two adjacent ones of the intersections, the travel distance between them, and the degree of fuel efficiency with which the vehicle travels over each section, stores the time, the travel distance, and the degree of fuel efficiency as a database for each travel, for enabling a search for a route which provides the shortest time, a route which provides the shortest distance, or a route with the highest degree of fuel efficiency by using those data, and for displaying the route searched for.